Tax write-off. He knows the shares are worthless, so might as well realize the loss and use it to offset gains elsewhere.
That makes no sense. Effective tax management means finding ways to report every possible loss, not creating actual losses just so you can report them. Creating $100 in actual losses to offset $100 in gains elsewhere lowers your tax liability by somewhere between $15 and $40, depending, which means you're actually throwing away $60-$85 in the process. Better to keep the gain and pay the tax.
There may be reasons to want to realize the loss *now*, rather than in the future, but that could have been done by selling the shares for more money -- assuming buyers could be found. Perhaps Murdoch believes that no one would be willing to buy his shares for more money? That seems unlikely. Hell, I'll give him $2. I have no reason to believe that the shares are worth that much, but the odds that they are are probably higher than the odds that I'm going to win the lottery and I have bought a lottery ticket a time or two.
Something else is going on here. Perhaps Murdoch has a personal friendship with Holmes, or some other non-financial motivation. But it makes no sense to artificially inflate your real losses in order reduce your tax liability, because the reduction in tax liability will always be less than the losses.